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15.6.5. 15.6.5. Vloeibare CO₂

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Auteur : David Bogert

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Difficulty : Level 7

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There are a whole series of products which supposedly add “liquid carbon” to a planted aquarium. There are three points which summarize the discussion: To go into more depth read on. “Liquid Carbon” in Depth There is one supplier which is honest about “liquid carbon”. It is Cory at Aquarium Co-op. Cory is the most ... Lees meer

There are a whole series of products which supposedly add “liquid carbon” to a planted aquarium.

There are three points which summarize the discussion:

  • “Liquid carbon” does preferentially kill algae over vascular plants because the cell walls of algae are thinner than the cell walls of vascular plants.
  • Since the cell walls of a fish’s gills are even thinner than that of algae, “liquid carbon” kills cells in the gills of a fish …. whoops
  • “Liquid carbon” add only a TINY TINY bit of carbon dioxide to any aquarium

To go into more depth read on.

“Liquid Carbon” in Depth

There is one supplier which is honest about “liquid carbon”. It is Cory at Aquarium Co-op. Cory is the most knowledgeable and more importantly the ONLY honest expert in the business. What he says about “liquid carbon” is VERY instructive:

What is Liquid Carbon?
Liquid carbon, also known as liquid carbon dioxide or liquid CO₂, is offered by many aquarium companies as part of their plant fertilizer collection. You may see it sold in the United States under brand names like Seachem Flourish Excel, API CO₂ Booster, and our own Aquarium Co-Op Easy Carbon. Almost all of these products contain glutaraldehyde or similar chemicals, which is known to reduce algae growth within a planted aquarium. When there is less algae competing for nutrients, light, and carbon dioxide, aquatic plants often grow faster and healthier as a result.

Cory at Aquarium Co-op

Does liquid CO₂ replace pressurized CO₂ gas?
While liquid carbon is often marketed as a source of carbon, many planted tank experts agree that it is a poor replacement for CO₂ gas. People set up high tech tanks to help their underwater plants by injecting more CO₂ gas into the water at concentrations ranging from 10-30 ppm. Some initial studies have analyzed how much CO₂ is released from recommended doses of liquid carbon, and the numbers were significantly lower than pressurized CO₂. In fact, a normal aquarium with good surface agitation and gas exchange using an air stone or filter results in approximately 3-5 ppm CO₂ in the water, which is still higher than what liquid carbon appears to supply.

Cory at Aquarium Co-op

I cannot say it better than Cory says it. He is just right on. But others in the business are not so honest. Chief among these snake oil salesmen is Seachem.

Beplant aquarium
Beplant aquarium

“Liquid Carbon”

Any discussion of “liquid carbon” has to be predicated with a clarification.

“Liquid Carbon” is NOT liquid carbon.

“Liquid Carbon” products are an aldehyde, an organic compound that has carbon, hydrogen and oxygen atoms in its molecular structure. In theory, when the “liquid carbon” is metabolized by bacteria, it will produce very tiny amounts of carbon dioxide gas which plants can utilize. But the amount of carbon dioxide produced is negligible.

High Tech Planted Aquarium
High Tech Planted Aquarium

Flourish Excel

Flourish Excel supposedly “adds carbon” to the planted aquarium.

“Excel™ is an exclusive Seachem trade secret. It is based on many years of in house testing and research that shows a tremendous increase in growth similar in nature to that seen when CO₂ is used. Polycycloglutaracetal is an isomeric form of glutaraldehyde… however it is less reactive and more easily utilized by plants as a carbon source. Once we have patented the product we would then be in a position to release detailed scientific studies (for trade secret reasons until then of course).”


To a chemist this is hilarious. This is just the “pseudo-scientific bedazzlement” so characteristic of Seachem with all its products. It is gibberish. All green plants and algae use carbon dioxide as their sole carbon source. There is no pathway by which an organic carbon compound can be utilized directly as a food source by green plants. If there were such a pathway sugar, organic acids (citric, acetic, etc.) or alcohol would provide the direct “carbon source” much more safely. Have you ever fertilized your garden with sugar?

Well Planted Aquarium
Well Planted Aquarium

Note that Seachem could easily release the results of their “detailed scientific studies” without revealing what their “Polycycloglutaracetal” is. But they don’t do that. Indeed, there isn’t a single study that can be found anywhere on Seachem’s website supporting any of its ridiculous marketing hype. Nor is there any study in the scientific literature supporting their claim.

Now there are a host of YouTube videos where everyone gets very “scientific” and uses a lot of big words to support the claims made by Seachem about Excel. Thing like the “Calvin cycle” etc. Take it from a genuine scientist, it is ALL just bull_____ .  There are even some “side by side” comparisons where tanks had great plant growth with Excel and little plant growth without it. Understand, Seachem does a lot of “sponsoring” of YouTube makers. Don’t believe a word of it.

Beplant aquarium
Beplant aquarium

As a chemist let me assure you there is no such thing as “Polycycloglutaracetal”. It is possible for glutaraldehyde to do some polymerization but so what? Even low molecular weight polyglutaracetal would be insoluble in water and it would present as a white precipitate.  Polymerized glutaraldehyde simply cannot be absorbed by plants and can do nothing to help the plants.

Excel is simply a solution of glutaraldehyde.  Glutaraldehyde is a simple organic carbon containing compound C5H8O₂. Like many simple organic compounds, it is decomposed into carbon dioxide by bacteria in the filter of the aquarium.

Beplant aquarium
Beplant aquarium

While glutaraldehyde does contain a minimal amount of carbon, using a standard dose, it releases very roughly 1 ppm of carbon dioxide via bacterial oxidation over twenty four hours. This would only release very roughly 0.04 ppm per hour. Atmospheric equilibrium from gaseous exchange provides roughly 1 ppm of carbon dioxide per hour at pretty much all times. So that means that doing absolutely nothing provides very roughly twenty five times more carbon dioxide than “liquid carbon”. So the “carbon” claim is a pure snake oil scam.

Note the excellent YouTube Channel Aquapros did a test of “liquid carbon” and found it simply did not work. It did not increase the growth of plants.

Planted aquarium
Planted aquarium

Wikipedia (sometime Wikipedia is correct!) gives an accurate assessment of glutaraldehyde:

Glutaraldehyde, sold under the brandname Cidex and Glutaral among others, is a disinfectant, medication, preservative, and fixative. As a disinfectant, it is used to sterilize surgical instruments and other areas of hospitals. As a medication, it is used to treat warts on the bottom of the feet. Glutaraldehyde is applied as a liquid.
Side effects include skin irritation. If exposed to large amounts, nausea, headache, and shortness of breath may occur. Protective equipment is recommended when used, especially in high concentrations. Glutaraldehyde is effective against a range of microorganisms including spores.


This data is confirmed by the OSHA write-up on glutaraldehyde. Does this sound like something you want to expose your fish or plants to?

Glutaraldehyde is closely related to formaldehyde but is eight times more toxic than formaldehyde. Glutaraldehyde is a disinfectant. Like all disinfectants it kills all living cells it meets: algae, plants, beneficial bacteria and the gills of the fish. So adding it to an aquarium is a balancing act of not killing enough cells to kill the plants or the fish while killing the algae. I will not use it in any form in my aquariums.

Planted aquarium
Planted aquarium

Meer informatie over CO₂ systemen

General Information on CO₂ in the aquarium can be found at this link:

15.6. CO₂ in the Planted Aquarium